Atheism and agnosticism remain poorly understood, and confusion continues even among some who call themselves atheists or agnostics. This diagram (source unknown) puts all the pieces together perfectly and illustrates that the agnostic-gnostic dimension deals with certainty while the atheist-theist dimension focuses on belief and has nothing to do with certainty.
I am an atheist in that I lack belief in god(s). I usually identify myself as an agnostic atheist, but that depends on exactly how "god" or "gods" is defined. If a theist defines these terms in such an absurd way that they describe contradictory or incoherent notions, then I might be an gnostic atheist for that particular god because I'd know that an entity with logically incoherent properties does not exist because such a thing could not exist.